1. Field of the Invention
This invention relates to the field of electrolytes used in the electrochemical machining of nickel base superalloys.
2. Description of the Prior Art
Electrochemical machining (ECM) involves the controlled removal of metal from a workpiece by anodic dissolution in an electrolytic cell in which the workpiece is the anode and the tool is the cathode. The electrolyte is pumped through the gap between the tool and the workpiece while a direct current is passed through the cell at a low voltage so as to dissolve metal from the workpiece. The ECM process is analogous to reverse electroplating.
ECM can be used to do work that would be difficult or impossible to do by other means. The work possible by ECM includes machining of extremely hard materials and the machining of odd shaped and/or small deep holes. ECM is widely used in the gas turbine industry for the production of small holes for air cooling in gas turbine blades.
The electrolytes used in the past for the electrochemical machining usually involve aqueous solutions of inorganic salts such as sodium chloride, potassium chloride, sodium nitrate and sodium chlorate. Other electrolytes such as sulphuric acid and hydrochloric acid solutions have been used in certain instances.
The electrolyte has three main functions in the ECM process. It carries the current between the tool and the workpiece, it removes the product of the reaction from the cutting region, and it removes the heat produced by the current flow in the operation.
Electrolytes must have high conductivity, low toxicity and corrosivity, and chemical and electrochemical stability.
Sludge is a material formed during the ECM process and consists mainly of metal hydroxides and other reaction products. Under some conditions, as much as 100 cubic inches of sludge may be produced for each cubic inch of metal removed by the ECM process. Smut consists of extremely fine particles of alloy constituents, mainly metallic. Both sludge and smut are undesirable since they interfere with the ECM process and are difficult to remove from the finished part. As a general rule, solutions of inorganic salts such as sodium chloride are sludging electrolytes. the other general type of electrolyte is termed nonsludging. For example, solutions of strong acids tend to retain the anodically removed metal in solution and thus do not produce sludge under normal conditions. the use of solutions of mild inorganic acids as electrochemical machining electrolytes does not appear to be common. Most acids used in electrolytes for electrochemical machining are the strong mineral acids such as nitric, sulphuric, and hydrochloric acid.
Of course, many types of acid mixtures have been used as chemical etching agents without the assistance of electric current. For example, in U.S. Pat. No. 3,524,817 the use of nitric acid and citric acid for the chemical deburring of zinc is described. In U.S. Pat. No. 2,849,297 the use of nitric and hydrochloric acids for cleaning and polishing magnesium is described. U.S. Pat. No. 3,753,817 describes the use of a mixture of nitric, hydrochloric and acetic acids for use in etching nickel alloy wire. U.S. Pat. No. 3,709,824 describes the use of phosphoric, nitric and hydrochloric acids for the polishing of stainless steel surfaces. In U.S. Pat. No. 3,275,560 the use of hydrochloric and nitrix acid mixtures for cleaning various metals is described. None of these preceding U.S. patents involves the use of such acid mixtures in electrochemical machining.